Monolithic integrated coherent transceiver

ABSTRACT

Various embodiments of a monolithic transceiver are described, which may be fabricated on a semiconductor substrate. The monolithic transceiver includes a coherent receiver module (CRM), a coherent transmitter module (CTM), and a local oscillation splitter to feed a local oscillation to the CRM and the CTM with a tunable power ratio. The monolithic transceiver provides tunable responsivity by employing avalanche photodiodes (APDs) for opto-electrical conversion. The monolithic transceiver also employs a polarization beam rotator-splitter (PBRS) and a polarization beam rotator-combiner (PBRC) for supporting modulation schemes including polarization multiplexed quadrature amplitude modulation (PM-QAM) and polarization multiplexed quadrature phase shift keying (PM-QPSK).

CROSS-REFERENCE TO RELATED PATENT APPLICATIONS

The present disclosure claims the priority benefit of U.S. PatentApplication No. 62/919,999, filed on Apr. 8, 2019. The aforementionedapplication is incorporated by reference in its entirety.

TECHNICAL FIELD

The present disclosure relates to the technical field of opticaltransceivers. More particularly, the present disclosure relates to amonolithic integrated coherent transceiver having various parametertuning capabilities.

BACKGROUND

Contemporary telecommunication systems involve applications where alarge amount of data is transmitted at a high data rate across a fairlylong distance, such as long-hual data communication, metro datacommunication, and communication between data centers. For instance,datacenter interconnection (DCI) applications usually involvetransmission of data across a distance of 10-120 kilometers (km), at adata rate of 100 gigabits per second (Gb/s) and beyond. To this end,fiber-based coherent optical communication technologies are oftenemployed. In order to realize the high data rate, a transceiver forcoherent optical communication is required to modulate both theamplitude and the phase of an optical carrier to generate an opticalsignal, as well as demodulate such an optical signal. Moreover, theoptical signal is often transmitted across multiple polarizationsallowed by the transmission medium, e.g., across both the transverseelectric (TE) polarization and the transverse magnetic (TM) polarizationof a single-mode fiber.

In addition to the considerably larger amount of data being transmittedand received, contemporary telecommunication systems also require atransceiver to provide various economical and operational benefits suchas low cost, low power, and a small footprint. For example, transceiversmay be demanded to conform to an Integrated CoherentTransmitter-Receiver Optical Subassembly (IC-TROSA) packaging forlong-hual data communications, or a C form-factor pluggable version 2(CFP2) packaging for metro data communications. For DCI applications,various compact housing form factors have become standards, such asOptical Small Form Pluggable (OSFP), Quad Small Form Pluggable DoubleDensity (QSFP-DD), and Quad Small Form Pluggable Double Density 28(QSFP28). This in turn requires the optical sub-assemblies (OSAs) insidea transceiver to have very compact sizes. An integrated coherenttransmitter and receiver (ICTR) is a good choice of solution. An ICTR isa monolithically integrated optoelectrical chip fabricated on asemiconductor substrate, such as a silicon (Si) or silicon-on-insulator(SOI) substrate, using fabrication techniques similar to those employedin manufacturing semiconductor integrated circuits (ICs). While an ICTRmay fulfill requirements of low cost, low power and a small footprint,it also raises various challenges in its design, fabrication andoperation that a transceiver made of discrete components does not face.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings are included to provide a furtherunderstanding of the present disclosure, and are incorporated in andconstitute a part of this specification. The drawings illustrateembodiments of the present disclosure and, together with thedescription, serve to explain the principles of the present disclosure.The drawings may not necessarily be in scale so as to better presentcertain features of the illustrated subject matter. The left-mostdigit(s) of a reference number identifies the figure in which thereference number first appears. The use of the same reference numbers indifferent figures indicates similar or identical items.

FIG. 1 illustrates a schematic diagram of an integrated coherenttransmitter and receiver (ICTR) in accordance with an embodiment of thepresent disclosure.

FIG. 2 illustrates a cross-sectional view of a monolithic avalanchephotodiode (APD) in accordance with an embodiment of the presentdisclosure.

FIG. 3 illustrates a schematic diagram of an optical splitter inaccordance with an embodiment of the present disclosure.

FIG. 4 illustrates a schematic diagram of an optical splitter inaccordance with another embodiment of the present disclosure.

FIG. 5 illustrates a schematic diagram of a portion of an IQ modulatorin accordance with an embodiment of the present disclosure.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

In the following detailed description, numerous specific details are setforth by way of examples in order to provide a thorough understanding ofthe relevant teachings. Any variations, derivatives and/or extensionsbased on teachings described herein are within the protective scope ofthe present disclosure. In some instances, well-known methods,procedures, components, and/or circuitry pertaining to one or moreexample implementations disclosed herein may be described at arelatively high level without detail, in order to avoid unnecessarilyobscuring aspects of teachings of the present disclosure.

As described above, integrated coherent transmitters and receivers(ICTRs) pose various design, fabrication and operation challengesspecific to the monolithically fabricated transceivers. Variousparameters need to be tuned for individual transceivers, even forindividual components inside a transceiver, to overcome the challenges.In addition, the material system used to fabricate an ICTR needs to beselected strategically. Conventionally, an ICTR is fabricated using aIII-V compound material system, such as gallium arsenide (GaAs) orindium phosphide (InP). Albeit their superior electro-optical andopto-electrical properties (e.g., operating bandwidth and responsivityof a photodiode, propagation loss in a waveguide), III-V compoundmaterial systems generally have a fabrication cost that is much higherthan that of the more mature silicon basedcomplementary-metal-oxide-semiconductor (CMOS) fabrication processes,which have been widely used for manufacturing electrical integratedcircuits (ICs). In the present disclosure, a silicon based ICTRfabricated using a CMOS technology is presented. Among the manyadvantages of the silicon based ICTR, various parameter tuningcapabilities are built in for performance optimization. For example,avalanche photodiodes are employed in the silicon based ICTR in place ofconventional p-type-intrinsic-n-type (PIN) photodiodes for enhancing thedetection sensitivity, extending the operating bandwidth, as well astuning internal gain for individual photodiodes.

FIG. 1 illustrates a schematic diagram of an ICTR 100 fabricated on amonolithic semiconductor substrate 109. The monolithic substrate 109 maybe a silicon (Si) substrate or a silicon-on-insulator (SOI) substrate.The ICTR 100 includes a coherent receiver module (CRM) 104 and acoherent transmitter module (CTM) 105. The CRM 104 functions as areceiver of the ICTR 100, capable of extracting information embedded inan optical input received by the ICTR 100 via an input port 102 of theICTR 100. The optical input is a carrier (e.g., a high-frequencysinusoidal optical wave) modulated by a signal (e.g., a low-frequencyanalog signal or a low-frequency digital sequence) through a modulationscheme. The information extracted by the CRM 104 is presented in anelectrical form, such as electrical voltages or currents presented on aset of electrical pads 114. The CTM 105 functions as a transmitter ofthe ICTR 100, capable of encoding information into an optical outputsignal, which is transmitted from the ICTR 100 via an output port 103 ofthe ICTR 100. The information encoded into the optical output signal maybe received electrically by the ICTR 100 via another input port, whichmay encompass a set of electrical pads 115.

The ICTR 100 also includes a local oscillation (LO) splitter 130. The LOsplitter 130 has one input and two outputs. The input of the LO splitter130 is coupled with an optical waveguide 131, whereas the two outputs ofthe LO splitter 130 are coupled with optical waveguides 132 and 133,respectively. Each of the optical waveguides 131, 132 and 133 has twoopposing ends, and is used to transmit an optical signal between the twoopposing ends while the optical signal is substantially confined withinthe respective waveguide. As shown in FIG. 1, the optical waveguide 131is used to transmit an optical LO that is received by the ICTR 100 viaan input port 101 of the ICTR 100. Similarly, the optical waveguide 132is used to transmit an optical signal from the LO splitter 130 to theCRM 104, whereas the optical waveguide 133 is used to transmit anoptical signal from the LO splitter 130 to the CTM 105.

The optical LO received at the input port 101 may be a sinusoidaloptical wave, having a frequency that is substantially equal to thefrequency of the carrier of the optical input. The LO splitter 130splits the optical LO into a first LO and a second LO, with the first LOfed into the waveguide 132 and transmitted to the CRM 104, and thesecond LO fed into the waveguide 133 and transmitted to the CTM 105. TheLO splitter 130 may be programmed to split the optical LO into the firstLO and the second LO with a certain splitting ratio between the first LOand the second LO. For example, in an event that it is intended to haveequal power of the optical LO going to the CRM 104 and the CTM 105,respectively, the LO splitter 130 may be programmed to split the opticalLO such that the splitting ratio is 1:1. Namely, the first LO has halfof the total power of the optical LO, whereas the second LO has anotherhalf of the total power of the optical LO. As another example, in anevent that it is intended to have more of the optical LO power going tothe CRM 104 than that going to the CTM 105, the LO splitter 130 may beprogrammed to split the optical LO such that the splitting ratio is 3:1.Namely, the first LO has 75% of the total power of the optical LO,whereas the second LO has 25% of the total power of the optical LO.Specific mechanisms employed by the LO splitter 130 for setting thesplitting ratio are described further below.

In some embodiments, the LO splitter 130 may be a variable ratio opticalsplitter. That is, the splitting ratio between the first LO traveling tothe CRM 104 and the second LO traveling to the CTM 105 can be changed,and in some embodiments can even be tuned or otherwise adjusteddynamically, in a real-time fashion, as the ICTR 100 is operating toreceive and/or transmit optical signals. Being a variable ratio opticalsplitter, the LO splitter 130 provides a parameter tuning capability tooptimize performance of the ICTR 100. For example, the LO splitter 130is able to tune its splitting ratio to allocate more power in the secondLO, such that the CTR 105 is able to deliver more output power at theoutput port 103. As another example, the LO splitter 130 is able to tuneits splitting ratio to allocate more power in the first LO, such thatthe optical input entering the CRM 104 is way above a noise floor of theCRM 104.

The CRM 104 is configured to detect the signal embedded in the modulatedcarrier of the optical input received via the input port 102. In someembodiments, the carrier is modulated by the signal using a modulationscheme of polarization multiplexed quadrature amplitude modulation(PM-QAM) or polarization multiplexed quadrature phase shift keying(PM-QPSK). The CRM 104 detects the signal by feeding the optical inputand the first LO into hybrid mixers (HMs) 141 and 142 thereof. The CRM104 also includes a polarization beam rotator-splitter (PBRS) 144 toprocess an optical input that is using a polarization multiplexedmodulation scheme before feeding the optical input into HMs 141 and 142.Specifically, a polarization multiplexed optical input carriesinformation in more than one optical polarization. For example, theoptical input received at the input port 102 may employ a PM-QAM orPM-QPSK modulation scheme. That is, the optical input includes at leasttwo components, each of which is polarized at a respective opticalpolarization, such as a transverse electric (TE) component and atransverse magnetic (TM) component. The TE component is aQAM/QPSK-modulated carrier polarized in the TE polarization, whereas theTM component is a QAM/QPSK-modulated carrier polarized in the TMpolarization. The PBRS 144 separates the TE component from the TMcomponent so that they can be separately processed by the HM 141 and theHM 142, respectively, to detect the respective signal encoded in the TEand TM components. It is to be noted that, although the TE component andthe TM component are polarized differently in the optical input, theywould exhibit a same optical polarization after passing the PBRS 144,whereas the signals encoded in the TE and TM components are separated bythe PBRS 144 into two optical paths, subsequently processed by the HM141 and the HM 142, respectively. For instance, before passing the PBRS144, the TE component traveling in the waveguide 145 is physicallyhaving the TE polarization, whereas the TM component traveling in thewaveguide 145 is physically having the TM polarization. After passingthe PBRS 144, however, the TM component traveling in the waveguide 147is rotated to be physically in the TE polarization, whereas the TEcomponent traveling in the waveguide 146 remains physically in the TEpolarization.

In addition to the LO splitter 130, the CRM 104 has another opticalsplitter 136, which is used to split the first LO traveling in thewaveguide 132 into two branches and feed them to the HMs 141 and 142 viawaveguide 137 and 138, respectively. In some embodiments, the opticalsplitter 136 is a 3 dB coupler, which splits the first LO into the twobranches with equal power. Namely, the first LO fed into the HM 141 viathe waveguide 137 has the same optical power as the first LO fed intothe HM 142 via the waveguide 138. In some embodiments, the opticalsplitter 136 may be a variable ratio optical splitter, which is capableof programming or otherwise setting a splitting ratio between the firstLO traveling to the HM 141 and the first LO traveling to the HM 142 in away similar to how the LO splitter 130 sets the splitting ratio betweenthe first LO traveling to the CRM 104 and the second LO traveling to theCTM 105. The variable ratio optical splitter 136 provides a parametertuning capability to optimize performance of the CRM 104. For example,being a variable ratio splitter, the optical splitter 136 is able totune its splitting ratio to compensate for a systematic offset, if any,between the HM 141 and the HM 142.

More parameter tuning capabilities are built into the CRM 104 by theinclusion of variable optical attenuators (VOAs) 151, 152 and 153, aswell as monitoring photodiodes (MPDs) 154, 155 and 156. A VOA is anin-line attenuator, which is capable of providing a programmableattenuation in power for an optical signal passing through the VOA.Namely, the VOAs 151, 152 and 153 provide certain power attenuations tothe optical waves traveling in the waveguides 146, 147 and 132,respectively. An MPD is a photodiode connected with an optical coupler.The coupler is used to tap into a waveguide carrying an optical wave bycoupling a predetermined small amount of the optical signal to thephotodiode. A reading of the photodiode is thus an indication of thepower of the optical wave. As shown in FIG. 1, MPDs 154, 155 and 156 areused to monitor a power of the optical wave traveling in the waveguides146, 147 and 132, respectively. Accordingly, a reading of the MPD 154provides an indication of the optical power of the TE component of theoptical input received via the input port 102. Similarly, a reading ofthe MPD 155 provides an indication of the optical power of the TMcomponent of the optical input, whereas a reading of the MPD 156provides an indication of the optical power of the first LO after thefirst LO is split from the optical LO by the LO splitter 130. Accordingto the readings of the MPDs 154, 155 and 156, one or more of the VOAs151, 152 and 153 may be tuned to provide certain attenuations to the TEcomponent, the TM component, and the first LO, respectively, so that theHMs 141 and 142 may have substantially equal optical power at theirinputs. In some embodiments, the attenuations provided by the VOAs 151,152 and 153 may be further tuned to compensate for a systematic offset,if any, between the HM 141 and the HM 142.

Each of the HMs 141 and 142, having two inputs and four outputs, is a90-degree hybrid mixer. A 90-degree hybrid mixer is capable ofgenerating four respectively different beat phase offsets at a 90-degreeinterval based on the two inputs. Let E1 and E2 denote the opticalsignals at the two inputs, respectively, and let E3, E4, E5 and E6denote the optical signals at the two inputs, respectively. The90-degree hybrid mixer is functionally described using the followingequations:E3=E1−E2;E4=E1+E2;E5=E1−jE2;E6=E1+jE2.

Namely, each of E3, E4, E5 and E6 is a predefined beat phase offsetbased on E1 and E2. Specifically, E3 is a predefined beat phase offsetof 180 degrees based on E1 and E2; E4 is a predefined beat phase offsetof 0 degrees based on E1 and E2; E5 is a predefined beat phase offset of−90 degrees based on E1 and E2; and E6 is a predefined beat phase offsetof 90 degrees based on E1 and E2. Accordingly, the HM 141 generates atits outputs predefined beat phase offsets of 0 degrees, 180 degrees, 90degrees and −90 degrees based on the first LO that arrives at the HM 141via the waveguide 137 and TE component that arrives at the HM 141 viathe waveguide 146. Similarly, the HM 142 generates at its outputspredefined beat phase offsets of 0 degrees, 180 degrees, 90 degrees and−90 degrees based on the first LO that arrives at the HM 142 via thewaveguide 138 and TM component that arrives at the HM 142 via thewaveguide 147. The outputs of the HMs 141 and 142 are coupled to anoptical detection array (ODA) 143 that has eight avalanche photodiodes(APDs). That is, each of the four outputs of the HM 141 and the fouroutputs of the HM 142 is respectively coupled to a corresponding one ofthe eight APDs of the ODA 143. The APDs of the ODA 143 are used toconvert the outputs of the HMs 141 and 142 from an optical form into anelectrical form, which may be manifested as electrical voltages orcurrents presented on the set of electrical pads 114.

The ODA 143 of the ICTR 100 employs APDs instead of conventionally usedPIN photodiodes so as to take advantage of the significant internalcurrent gain of the APDs. A PIN photodiode cannot provide an internalcurrent gain. In contrast, thanks to the avalanche mechanism of an APD,the opto-electrical (O-E) responsivity of an APD is significantly higheras compared to that of a PIN photodiode. This translates to a higher O-Eresponsivity of the ODA 143, and thus a higher sensitivity of the ICTR100, which is beneficial in that the higher sensitivity of the ICTR 100relaxes a minimum power requirement of the optical LO as received at theinput port 101. The optical LO is generally generated by another opticalsub-assembly (OSA), i.e., an integrable tunable laser assembly (ITLA),that is coupled to the input port 101. The high O-E responsivity of theODA 143 means that the ICTR 100 can work with a less powerful ITLA,which is a huge cost benefit, as a powerful ITLA is typically expensive.

Furthermore, the O-E responsivity of a PIN photodiode is fixed, i.e.,the O-E responsivity cannot be tuned or otherwise modified after itsfabrication. This poses a challenge for certain wavelengths of the ITLA.For example, while having a reasonable O-E responsivity in the C-band(i.e., the wavelength of the optical LO is in the range of 1530-1565nm), the conventional PIN photodiodes suffers a degradation in the O-Eresponsivity when ITLA operates in a longer wavelength range such asL-band (i.e., the wavelength of the optical LO is in the range of1565-1625 nm). The performance of a transceiver employing PINphotodiodes therefore suffers accordingly. In contrast, the internalcurrent gain of an APD is tunable via a bias voltage applied to the APD.The bias voltage can be tuned to increase the internal current gain ofthe APD for compensating for any degradation in the O-E responsivity dueto the APD operating in different wavelength range. For example, bytuning the bias voltage for the APDs in the ODA 143, any sensitivitydegradation of the ICTR 100 when operating in the L-band may berecovered so that the ICTR 100 has comparable sensitivity across C-bandand L-band. This is another parameter tuning capability of the ICTR 100.

In some embodiments, the internal current gain for each of the eightAPDs of the ODA 143 may be individually programmable by adjusting arespective bias voltage for the APD. This capability to finetuneperformance of individual APDs of the ODA 143 provides a way tocompensate for any random and/or systematic offset that could arise inthe fabrication process of the ODA 143. As a result, the eight APDs ofthe ODA 143 can be tuned to have a better uniformity, which translatesto an improved electrical common mode rejection ratio (CMRR) across theeight APDs.

FIG. 2 illustrates a cross-sectional view of a monolithic APD 200 thatmay be employed to realize the APDs of the ODA 143. The APD 200 isfabricated using a CMOS-compatible manufacturing process. Specifically,the manufacturing of the APD 200 starts with a silicon substrate 212. Aninsulator layer 210, such as a layer of silicon dioxide, is disposed ontop of the substrate 212. A bottom contact layer 211 is disposed on topof the insulator layer 210. The bottom contact layer 211 is made ofsilicon doped with a n-type dopant, such as arsenic (As) or phosphorus(P), at a doping concentration in a range of 5e18-5e19 per cubiccentimeter (cm⁻³). The APD 200 further includes a n++ silicon region 221that is made of silicon doped with the n-type dopant at a dopingconcentration higher than that of the bottom contact layer 211, and thusprovides a satisfactory ohmic contact between an electrical conductor218 and the bottom contact layer 211. The APD 200 also includes an-charge layer 213. The n-charge layer 213 is made of silicon doped withthe n-type dopant at a doping concentration in a range of 2e17-5e18cm⁻³.

Additionally, an avalanche layer 215, made of intrinsic silicon, isdisposed on top of the bottom contact layer 211. A p-charge layer 216 isdisposed on top of the avalanche layer 215. The p-charge layer 216 ismade of moderately doped p-type silicon, i.e., silicon doped with ap-type dopant such as boron (B). The p-charge layer 216 is doped at adoping concentration in a range of 8e16-2e18 cm⁻³. In some embodiments,The APD 200 may include a guard ring 214 surrounding the p-charge layer216. The guard ring 214 is made of silicon doped with the p-type dopantat a doping concentration higher than that of the p-charge layer 216.

Additionally, an absorption layer 219, made of intrinsic germanium, isdisposed on top of the p-charge layer 216. A top contact layer 220 isdisposed on top of the absorption layer 219. The top contact layer 220is made of heavily doped p-type amorphous silicon. Specifically, the topcontact layer 220 comprises amorphous silicon (a-Si) doped with thep-type dopant at a doping concentration in a range of 5e18-5e19 cm⁻³. Anelectrical conductor 228 is disposed on top of the top contact layer220. Finally, a passivation material, such as silicon dioxide, isdeposited to form a passivation 223.

Through electrical conductors 218 and 228, a bias voltage may be appliedacross the top and bottom contact layers 220 and 211 to tune theinternal current gain of the APD 200. Specifically, an optical signal222 traveling in a waveguide 217 couples to the absorption layer 219 byway of evanescent fields. The evanescent fields in the absorption layer219 thus induce photo-carriers by photon adsorption, which subsequentlydrift into the avalanche layer 215. The bias voltage applied across thetop and bottom contact layers 220 and 211 determines an intensity of anelectric field in the avalanche layer 215. The photo-carriers that driftinto the avalanche layer 215 are accelerated by the electric field,resulting in an avalanche process that generates a photocurrent as anoutput of the APD 200. The ratio of resulted photocurrent to the inducedphoto-carriers is defined as the internal current gain of the APD 200.The higher the bias voltage, the stronger the electric field in theavalanche layer 215, and the larger the internal current gain.Therefore, the internal current gain is programmable by adjusting thebias voltage applied across the top and bottom contact layers 220 and211.

In some embodiments, APD 200 may be fabricated on an SOI substrate. Theinsulator layer 210 is realized by a buried oxide (BOX) layer of the SOIsubstrate, whereas the waveguide 217, the bottom contact layer 211, andthe n++ silicon region 221 are formed in a top silicon layer of the SOIsubstrate.

FIG. 3 illustrates a schematic diagram of an optical splitter 300 thatmay be used to realize either or both of the LO splitter 130 and theoptical splitter 136. The optical splitter 300 includes a 1×2 opticalcoupler 310, which has an input, a first output, and a second output.The optical splitter 300 also includes a phase shifter 320, which has aninput and an output. The phase shifter 320 is used to provide a phaseshift to the optical signal passing the phase shifter 320. In someembodiments, the phase shifter 320 comprises a heater, such as aresistive heating element. An electrical voltage is applied across theheater; an electrical current induced by the electrical voltage heats upthe heater and induces the phase shift. The optical splitter 300 furtherincludes a 2×2 optical coupler 330, which has a first input, a secondinput, a first output, and a second output.

The 1×2 optical coupler 310, the phase shifter 320, and the 2×2 opticalcoupler 330 are coupled to each other in a Mach-Zehnder interferometer(MZI) configuration. Specifically, the first output of the 1×2 opticalcoupler 310 is coupled to the input of the phase shifter 320, and theoutput of the phase shifter 320 is coupled to the first input of the 2×2optical coupler 330. The second output of the 1×2 optical coupler 310 iscoupled to the second input of the 2×2 optical coupler 330. Due to theMZI configuration, the phase shift provided by the phase shifter 320determines the splitting ratio of an optical signal entering the inputof the 1×2 optical coupler 310, as the optical signal is split andpresented at the first and second outputs of the 2×2 optical coupler330.

In some embodiments, the phase shifter 320 is a tunable phase shifter.That is, the electrical voltage applied across the heater of the phaseshifter 320 is adjustable. By adjusting the electrical voltage, thephase shifter 320 is able to provide different amount of phase shift,thereby changing the splitting ratio of the optical splitter 300.

In an event that the LO splitter 130 is realized by the optical splitter300, the 1×2 optical coupler 310 is used to receive the optical LO ofFIG. 1 at the input of the 1×2 optical coupler 310. The 2×2 opticalcoupler 330 is used to generate the first LO of FIG. 1 at the firstoutput of the 2×2 optical coupler 330, as well as the second LO of FIG.1 at the second output of the 2×2 optical coupler 330. In an event thatthe phase shifter 320 is a tunable phase shifter, the splitting ratiobetween the first LO and the second LO is determined by the phase shiftprovided by the phase shifter 320 based on the tunable electricalvoltage applied across the heater of the phase shifter 320.

FIG. 4 illustrates a schematic diagram of an optical splitter 400 thatmay be used to realize either or both of the LO splitter 130 and theoptical splitter 136. The optical splitter 400 is identical to theoptical splitter 300 except that the 1×2 optical coupler 310 is replacedby a combination of a PBRS 405 and a 2×2 optical coupler 410.Specifically, in addition to the phase shifter 320 and the 2×2 opticalcoupler 330, the optical splitter 400 also includes the PBRS 405, whichhas an input, a first output, and a second output, as well as the 2×2optical coupler 410, which has a first input, a second input, a firstoutput, and a second output. As shown in FIG. 4, the 2×2 optical coupler410, the phase shifter 320, and the 2×2 optical coupler 330 are coupledto each other in an MZI configuration. Specifically, the first output ofthe first 2×2 optical coupler 410 is coupled to the input of the phaseshifter 320. The output of the phase shifter 320 is coupled to the firstinput of the 2×2 optical coupler 330. The second output of the 2×2optical coupler 410 is coupled to the second input of the 2×2 opticalcoupler 330. In addition, the first output of the PBRS 405 is coupled tothe first input of the first 2×2 optical coupler 410, whereas the secondoutput of the PBRS 405 is coupled to the second input of the first 2×2optical coupler 410. Due to the MZI configuration of the opticalsplitter 400, the phase shift provided by the phase shifter 320determines the splitting ratio of an optical signal entering the inputof the PBRS 405, as the optical signal is split and presented at thefirst and second outputs of the 2×2 optical coupler 330.

In an event that the LO splitter 130 is realized by the optical splitter400, the PBRS 405 is used to receive the optical LO of FIG. 1 at theinput of the PBRS 405. The 2×2 optical coupler 330 is used to generatethe first LO of FIG. 1 at the first output of the 2×2 optical coupler330, as well as the second LO of FIG. 1 at the second output of the 2×2optical coupler 330. In an event that the phase shifter 320 of theoptical splitter 400 is a tunable phase shifter, the splitting ratiobetween the first LO and the second LO is determined by the phase shiftprovided by the phase shifter 320 based on the tunable electricalvoltage applied across the heater of the phase shifter 320.

An advantage of using the optical splitter 400 to realize the LOsplitter 130, as compared to using the optical splitter 300, resides inthat the LO splitter 130 realized by the optical splitter 400 isinsensitive to the polarization of the optical LO. In an event that theoptical LO propagates in the TM polarization when received at the inputport 101 of the ICTR 100, the PBRS 405 is able to rotate the optical LOto the TE polarization and feed the optical LO to both inputs of the 2×2optical coupler 410 with the TE polarization, resulting in the first LOand the second LO both being correctly polarized in the TE polarization.In contrast, the optical splitter 300 would result in the first LO andthe second LO being incorrectly polarized in the TM polarization in anevent that the optical LO propagates in the TM polarization whenarriving at the input port 101.

Refer back to FIG. 1. As described elsewhere in the present disclosure,the CTM 105 of the ICTR 100 is configured to generate an optical outputsignal based on the second LO generated by the LO splitter 130, as wellas the electrical modulation signal received via the set of electricalpads 115. The electrical modulation signal represents information to beencoded in the optical output signal. In some embodiments, theelectrical modulation signal comprises a set of PM-QAM or PM-QPSKsignals. The CTM 105 includes two IQ modulators 160 and 161. In an eventthat the electrical modulation signal comprises a set of PM-QAM orPM-QPSK signals, half of the information represented by the electricalmodulation signal is encoded by the IQ modulator 160, whereas the otherhalf of the information is encoded by the IQ modulator 161.Specifically, the IQ modulator 160 is configured to modulate the secondLO based on a first half of information represented by the electricalmodulation signal, whereas the IQ modulator 161 is configured tomodulate the second LO based on a second half of information representedby the electrical modulation signal. The second LO, as modulated by theIQ modulator 160, constitutes a first portion of the optical outputsignal presented at the output port 103 of the ICTR 100. Similarly, thesecond LO, as modulated by the IQ modulator 161, constitutes a secondportion of the optical output signal. As shown in FIG. 1, preferably,the IQ modulators 160 and 161 are mirrored images of one another interms of physical layout on the monolithic substrate 109.

The CTM 105 also includes a polarization beam rotator-combiner (PBRC)162. The PBRC 162 may be identical to the PBRS 130 except that the inputand output are reversed. The PBRC 162 is configured to combine the firstportion of the optical output signal, as generated by the IQ modulator160, and the second portion of the optical output signal, as generatedby the IQ modulator 161, into the optical output signal presented at theoutput port 103. Specifically, the first portion of the optical outputsignal is combined by the PBRC 162 as a TE component of the opticaloutput signal, whereas the second portion of the optical output signalis combined by the PBRC 162 as a TM component of the optical outputsignal.

Each of the IQ modulators 160 and 161 includes two Mach-Zehndermodulators (MZMs), referred as an I-arm 111 and a Q-arm 112. Preferably,in terms of physical layout on the monolithic substrate 109, the I-arm111 and the Q-arm 112 are mirrored images of one another, as shown inFIG. 1. Each of the I-arm 111 and the Q-arm 112 has a dual-drivepush-pull configuration, wherein the dual-drive push-pull configurationhas two serpentine optical paths, such as optical paths 117 and 118.Preferably, in terms of physical layout on the monolithic substrate 109,the two serpentine optical paths 117 and 118 are mirrored images of oneanother, as shown in FIG. 1.

Accordingly, the CTM 105 has two IQ modulators 160 and 161, each of thetwo IQ modulators 160 and 161 having two MZMs 111 and 112, each of thetwo MZMs 111 and 112 having two optical paths each in a form of aserpentine waveguide. This amounts to a total number of eight serpentineoptical paths in the CTM 105, as shown in FIG. 1. A plurality of 3 dBcouplers, i.e., 3 dB couplers 181, 182, 183, 184, 185, 186 and 187, areincluded in the CTM 105. The 3 dB couplers 181-187 are connected byoptical waveguides in three layers of 1-to-2 configuration so that theycollectively distribute the second LO in the waveguide 133 to each ofthe eight serpentine optical paths of the CTM 105 with substantiallyidentical optical power in each path. Namely, each of the eightserpentine optical paths has ⅛ of the second LO traveling inside.

Each of the two MZMs, i.e., the I-arm 111 and the Q-arm 112, furthercomprises a 3 dB coupler that is configured to combine the twoserpentine optical paths. For example, the I-arm 111 of the IQ modulator160 has a 3 dB coupler 188 to combine the two serpentine optical paths117 and 118. Moreover, each of the serpentine optical paths 117 and 118has an in-line phase tuner. As shown in FIG. 1, the serpentine opticalpath 117 has an in-line phase tuner 190, whereas the serpentine opticalpath 118 has an in-line phase tuner 191. In some embodiments, thein-line phase tuners 190 and 191 may be tunable phase shifters, whichmay be same as or similar to the one used as the tunable phase shifters320 of FIG. 3 and FIG. 4. The in-line phase tuners 190 and 191 arecollectively tuned so that a phase difference between the two serpentineoptical paths 117 and 118 is tuned to substantially zero.

In addition to the I-arm 111 and the Q-arm 112, each of the IQmodulators 160 and 161 also includes a 3 dB coupler (e.g., 3 dB coupler189), an MPD (e.g., MPD 164), as well as an IQ phase tuner (e.g., acombination of in-line phase tuners 192 and 193). The 3 dB coupler isconfigured to combine an output of the I-arm 111 and an output of theQ-arm 112 into an output of the IQ modulator, referred as “IQ output”.The MPD is configured to monitor a power of the IQ output, whereas theIQ phase tuner is configured to tune a phase difference between theI-arm output and the Q-arm output to substantially 90 degrees. The IQphase tuner, such as the combination of the in-line phase tuners 192 and193, may include one or more heating elements that can provide a tunablephase shift like how the tunable phase shifters 320 does. The IQ phasetuner is configured to tune the phase difference between the I-armoutput and the Q-arm output to substantially 90 degrees based on areading of the MPD, which indicates the power of the IQ output. Forexample, the IQ modulator 160 includes, in addition to the I-arm 111 andthe Q-arm 112, the 3 dB coupler 189, the MPD 164, as well as the IQphase tuner comprising the in-line phase tuners 192 and 193. The 3 dBcoupler 189 is used to combine the output of the I-arm 111 and theoutput of the Q-arm 112 into the IQ output of the IQ modulator 160, andthe MPD 164 is used to sense or otherwise monitor the IQ output. Thein-line phase tuners 192 and 193 are collectively tuned, based on thereading of the IQ output sensed by the MPD 164, to achieve a 90-degreephase difference between the output of the I-arm 111 and the output ofthe Q-arm 112.

FIG. 5 illustrates a schematic diagram of a portion of the IQ modulator160. Specifically, FIG. 5 illustrates a schematic diagram of the I-arm111 with more details. All MZMs of the CTM 105 share the same schematicdetails as illustrated in FIG. 5.

As shown in FIG. 5, the serpentine optical path 117 includes a pluralityof p-n junction diodes 517. The p-n junction diodes 517 are electricallyconnected in parallel with each other via electrical connectors 533, 534and 535, with the electrical connectors 533 and 535 electrically coupledto one another. Moreover, the I-arm 111 includes an electrical pad 576,which is used to receive a direct-current (DC) bias voltage V_(BIAS).The p-n junction diodes 517 are thus reversely biased by the DC biasvoltage V_(BIAS). Similarly, the serpentine optical path 118 includes aplurality of p-n junction diodes 518. The p-n junction diodes 518 areelectrically connected in parallel with each other via electricalconnectors 531, 532 and 533, with the electrical connectors 531 and 533electrically coupled to one another. The p-n junction diodes 518 arealso reversely biased by the DC bias voltage V_(BIAS). The I-arm 111includes electrical pads 572 and 574. At least a portion of theelectrical modulation signal received by the set of electrical pads 115are DC-coupled to the electrical conductors 532 and 534 via theelectrical pads 572 and 574. Whereas one end of the electrical conductor532 is coupled to the electrical pad 572, the other end of theelectrical conductor 532 is coupled to a load resistor 578. Likewise,one end of the electrical conductor 534 is coupled to the electrical pad574, whereas the other end of the electrical conductor 534 is coupled toa load resistor 579. In addition, the I-arm 111 includes six decouplingcapacitors 580, 581, 582, 583, 584 and 585 for providing decoupling forthe DC bias voltage V_(BIAS). Depending on a DC voltage level on theelectrical pads 572 and 574, a value of the DC bias voltage V_(BIAS) maybe changed accordingly so that the p-n junction diodes 517 and 518 aremaintained in a desired reverse bias region for an optimal performanceof the MZM 111.

Refer again to FIG. 1. When the ICTR 100 is used in a communicationsystem, the various optical ports of the ICTR 100 have to interface withother OSAs of the communication system. For example, the input port 101is to be aligned with an optical fiber cable coming from an ITLA. Theinput port 102 is to be aligned with an optical fiber cable that carriesthe optical input from possibly another transceiver to the CRM 104. Theoutput port 103 is to be aligned with an optical fiber cable thattransmits the optical output from the CTM 105 possibly to yet anothertransceiver. Therefore, it is essential for the ICTR 100 to havemechanisms that can ensure satisfactory fiber-to-chip alignment. Asshown in FIG. 1, the ICTR 100 includes two U-shaped edge couplerstructures 134 and 135, strategically placed at an edge of the substrate109 at a location close to the optical ports 101, 102 and 103. TheU-shaped edge coupler structures 134 and 135 are configured to helpfiber-to-chip alignment between the ICTR 100 and a 7-core fiber arraythat includes a fiber to be aligned with the input port 101 for sendingin the optical LO, a fiber to be aligned with the input port 102 forsending in the optical input, a fiber to be aligned with the output port103 for taking out the optical output, as well as four alignment fibersto be aligned with the U-shaped edge coupler structures 134 and 135 forsending and taking out two optical alignment beacons. In addition, theICTR 100 employs one or more MPDs to assist fiber-to-chip alignment. Forexample, the CTM 105 may include an MPD 163. During an alignmentprocedure of an optical fiber that is intended to be aligned with theICTR 100 at the output port 103, an alignment signal of a known opticalpower level may be sent from the optical fiber to the output port 103.The MPD 163 can thus be used to detect the alignment signal entering theICTR 100 via the output port 103. With the optical power level being aknown value, as well as the tap ratio of the optical coupler of the MPD163, an expected value of the optical power of the alignment signal thatis supposed to be detected by the MPD 163 can be calculated or otherwiseestimated. A reading of the MPD 163 that is close to the expected valuethus indicates a satisfactory alignment between the optical fiber andthe output port 103. On the other hand, a reading of the MPD 163 that issignificantly less than to the expected value may indicate anunsatisfactory alignment between the optical fiber and the output port103, The alignment procedure may be repeated until a satisfactoryalignment between the optical fiber and the output port 103 is achieved.

In some embodiments, any one of the MPDs of the ICTR 100, such as theMPD 154, the MPD 155, the MPD 156, the MPD 163, or the MPD 164, mayinclude an additional photodiode. That is, any one of the MPDs may havetwo photodiodes: a main photodiode that is connected with the opticalcoupler of the MPD, and a “dark” photodiode that is not connected withthe optical coupler. The main photodiode receives the optical input ofthe MPD through the optical coupler and thus generates a photocurrent,whereas the “dark” photodiode does not receive an optical input and thusonly generates a dark current but not a photocurrent. Preferably, themain photodiode and the “dark” photodiode are manufactured to bephysically identical and within close proximity on the substrate 109, sothat the dark current generated by the “dark” photodiode is a closereplica of the dark current of the main photodiode. The two photodiodesare configured in a so-called “balanced mode” configuration so that thedark current from the “dark” photodiode is used to calibrate out thedark current of the main photodiode, thereby achieving a highermeasurement accuracy of the optical power intended to be monitored bythe MPD.

The embodiments described herein provide technical advantages thatenable an ICTR having various parameter tuning capabilities, e.g., atunable gain for individual APDs of the ODA 143, a tunable power ofoptical input and local oscillation entering the HMs 141 and 142, atunable splitting ratio of local oscillation between the CRM 104 and CTM105, and tunable phase difference between various optical paths in theCTM 105. The parameter tuning capabilities of the ICTR as describedherein provide useful mechanisms to achieve optimized and more robustperformances of the ICTR, such as a wider operating range in terms ofwavelength and power, a more uniform responsivity, a higher common-moderejection as well as a higher tolerance to random noise sources insidethe ICTR. The ICTR with parameter tuning capabilities, as described inthe present disclosure, enables the optical telecommunication to becomemore efficient and cost effective.

Additional Notes

The herein-described subject matter sometimes illustrates differentcomponents contained within, or connected with, different othercomponents. It is to be understood that such depicted architectures aremerely examples, and that in fact many other architectures can beimplemented which achieve the same functionality. In a conceptual sense,any arrangement of components to achieve the same functionality iseffectively “associated” such that the desired functionality isachieved. Hence, any two components herein combined to achieve aparticular functionality can be seen as “associated with” each othersuch that the desired functionality is achieved, irrespective ofarchitectures or intermediate components. Likewise, any two componentsso associated can also be viewed as being “operably connected”, or“operably coupled”, to each other to achieve the desired functionality,and any two components capable of being so associated can also be viewedas being “operably couplable”, to each other to achieve the desiredfunctionality. Specific examples of operably couplable include but arenot limited to physically mateable and/or physically interactingcomponents and/or wirelessly interactable and/or wirelessly interactingcomponents and/or logically interacting and/or logically interactablecomponents.

Further, with respect to the use of substantially any plural and/orsingular terms herein, those having skill in the art can translate fromthe plural to the singular and/or from the singular to the plural as isappropriate to the context and/or application. The varioussingular/plural permutations may be expressly set forth herein for sakeof clarity.

Moreover, it will be understood by those skilled in the art that, ingeneral, terms used herein, and especially in the appended claims, e.g.,bodies of the appended claims, are generally intended as “open” terms,e.g., the term “including” should be interpreted as “including but notlimited to,” the term “having” should be interpreted as “having atleast,” the term “includes” should be interpreted as “includes but isnot limited to,” etc. It will be further understood by those within theart that if a specific number of an introduced claim recitation isintended, such an intent will be explicitly recited in the claim, and inthe absence of such recitation no such intent is present. For example,as an aid to understanding, the following appended claims may containusage of the introductory phrases “at least one” and “one or more” tointroduce claim recitations. However, the use of such phrases should notbe construed to imply that the introduction of a claim recitation by theindefinite articles “a” or “an” limits any particular claim containingsuch introduced claim recitation to implementations containing only onesuch recitation, even when the same claim includes the introductoryphrases “one or more” or “at least one” and indefinite articles such as“a” or “an,” e.g., “a” and/or “an” should be interpreted to mean “atleast one” or “one or more;” the same holds true for the use of definitearticles used to introduce claim recitations. In addition, even if aspecific number of an introduced claim recitation is explicitly recited,those skilled in the art will recognize that such recitation should beinterpreted to mean at least the recited number, e.g., the barerecitation of “two recitations,” without other modifiers, means at leasttwo recitations, or two or more recitations. Furthermore, in thoseinstances where a convention analogous to “at least one of A, B, and C,etc.” is used, in general such a construction is intended in the senseone having skill in the art would understand the convention, e.g., “asystem having at least one of A, B, and C” would include but not belimited to systems that have A alone, B alone, C alone, A and Btogether, A and C together, B and C together, and/or A, B, and Ctogether, etc. In those instances where a convention analogous to “atleast one of A, B, or C, etc.” is used, in general such a constructionis intended in the sense one having skill in the art would understandthe convention, e.g., “a system having at least one of A, B, or C” wouldinclude but not be limited to systems that have A alone, B alone, Calone, A and B together, A and C together, B and C together, and/or A,B, and C together, etc. It will be further understood by those withinthe art that virtually any disjunctive word and/or phrase presenting twoor more alternative terms, whether in the description, claims, ordrawings, should be understood to contemplate the possibilities ofincluding one of the terms, either of the terms, or both terms. Forexample, the phrase “A or B” will be understood to include thepossibilities of “A” or “B” or “A and B.”

Although the subject matter has been described in language specific tostructural features and/or methodological acts, it is to be understoodthat the subject matter defined in the appended claims is notnecessarily limited to the specific features or acts described above.Rather, the specific features and acts described above are disclosed asexample forms of implementing the claims.

What is claimed is:
 1. A monolithic coherent transceiver, comprising: afirst input port to receive an optical input, the optical inputcomprising a carrier modulated by a signal through a modulation scheme;a second input port to receive an optical local oscillation (LO), afrequency of the optical LO substantially equal to a frequency of thecarrier; a LO splitter to split the optical LO into a first LO and asecond LO with a splitting ratio between the first LO and the second LO;a coherent receiver module (CRM) to detect the signal based on theoptical input and the first LO; a third input port to receive anelectrical modulation signal; a coherent transmitter module (CTM) togenerate an optical output signal based on the second LO and theelectrical modulation signal; and an output port to transmit the opticaloutput signal, wherein the modulation scheme comprises polarizationmultiplexed quadrature amplitude modulation (PM-QAM) or polarizationmultiplexed quadrature phase shift keying (PM-QPSK), wherein the CTMcomprises: a first IQ modulator configured to modulate, based on a firsthalf of information represented by the electrical modulation signal, thesecond LO into a first portion of the optical output signal; a second IQmodulator configured to modulate, based on a second half of theinformation represented by the electrical modulation signal, the secondLO into a second portion of the optical output signal; and apolarization beam rotator-combiner (PBRC) configured to combine thefirst portion of the optical output signal and the second portion of theoptical output signal into the optical output signal, wherein the firstportion of the optical output signal is combined as a transverseelectric (TE) component of the optical output signal, and wherein thesecond portion of the optical output signal is combined as a transversemagnetic (TM) component of the optical output signal, wherein each ofthe first and second IQ modulators comprises: an I-arm comprising afirst Mach-Zehnder modulator (MZM) and configured to generate an I-armoutput; a Q-arm comprising a second MZM and configured to generate aQ-arm output; a 3 dB coupler to combine the I-arm output and the Q-armoutput into an IQ output; a monitoring photodiode (MPD) to monitor apower of the IQ output; and an IQ phase tuner configured to tune, basedon a reading of the MPD, a phase difference between the I-arm output andthe Q-arm output so that the phase difference is substantially 90degrees, wherein each of the first and second MZMs comprises adual-drive push-pull configuration, and wherein the dual-drive push-pullconfiguration comprises two serpentine optical paths, and wherein eachof the two serpentine optical paths comprises a plurality of p-njunction diodes connected in parallel, and wherein each of the first andsecond MZMs comprises an electrical pad to receive a direct-current biasvoltage that biases the plurality of p-n junction diodes.
 2. Themonolithic coherent transceiver of claim 1, wherein the optical inputcomprises a transverse electric (TE) component and a transverse magnetic(TM) component, and wherein the CRM comprises a polarization beamrotator-splitter (PBRS) to separate the TE component from the TMcomponent.
 3. The monolithic coherent transceiver of claim 2, whereinthe CRM further comprises: an optical detection array (ODA) comprising aplurality of avalanche photodiodes (APDs); a first hybrid mixer havingtwo inputs and four outputs, the two inputs of the first hybrid mixerrespectively coupled to the TE component and the first LO, each of thefour outputs of the first hybrid mixer respectively coupled to acorresponding APD of the ODA; and a second hybrid mixer having twoinputs and four outputs, the two inputs of the second hybrid mixerrespectively coupled to the TM component and the first LO, each of thefour outputs of the second hybrid mixer respectively coupled to acorresponding APD of the ODA.
 4. The monolithic coherent transceiver ofclaim 3, wherein an internal current gain of each of the plurality ofAPDs is programmable by adjusting a respective bias voltage applied tothe APD.
 5. The monolithic coherent transceiver of claim 3, wherein eachof the plurality of APDs comprises: a substrate comprising silicon; aninsulator layer disposed on top of the substrate, the insulator layercomprising silicon dioxide; a bottom contact layer disposed on top ofthe insulator layer, the bottom contact layer comprising heavily dopedn-type silicon; an avalanche layer disposed on top of the bottom contactlayer, the avalanche layer comprising intrinsic silicon; a p-chargelayer disposed on top of the avalanche layer, the p-charge layercomprising moderately doped p-type silicon; an absorption layer disposedon top of the p-charge layer, the absorption layer comprising intrinsicgermanium; and a top contact layer disposed on top of the absorptionlayer, the top contact layer comprising heavily doped p-type amorphoussilicon.
 6. The monolithic coherent transceiver of claim 5, wherein: thebottom contact layer comprises silicon doped by a n-type dopant with adoping concentration in a range of 5e18-5e19 per cubic centimeter(cm⁻³), the p-charge layer comprises silicon doped by a p-type dopantwith a doping concentration in a range of 8e16-2e18 cm⁻³, the topcontact layer comprises amorphous silicon doped by the p-type dopantwith a doping concentration in a range of 5e18-5e19 cm⁻³, the n-typedopant comprises arsenic or phosphorus, and the p-type dopant comprisesboron.
 7. The monolithic coherent transceiver of claim 5, wherein aninternal current gain of each of the plurality of APDs is programmableby adjusting a respective bias voltage applied across the top and bottomcontact layers of the APD.
 8. The monolithic coherent transceiver ofclaim 3, wherein the CRM further comprises: a first variable opticalattenuator (VOA) capable of adjusting a power of the first LO before thefirst LO is coupled to each of the first hybrid mixer and the secondhybrid mixer; a second VOA capable of adjusting a power of the TEcomponent before the TE component is coupled to the first hybrid mixer;and a third VOA capable of adjusting a power of the TM component beforethe TM component is coupled to the second hybrid mixer.
 9. Themonolithic coherent transceiver of claim 3, wherein the CRM furthercomprises: a first monitoring photodiode (MPD) to monitor a power of thefirst LO; a second MPD to monitor a power of the TE component; and athird MPD to monitor a power of the TM component.
 10. The monolithiccoherent transceiver of claim 1, wherein the IQ phase tuner comprisesone or more heating elements.
 11. The monolithic coherent transceiver ofclaim 1, wherein each of the first and second MZMs further comprises a 3dB coupler to combine the two serpentine optical paths, and wherein eachof the two serpentine optical paths comprises an in-line phase tunerconfigured to tune a phase difference between the two serpentine opticalpaths to substantially zero.
 12. The monolithic coherent transceiver ofclaim 1, wherein the two serpentine optical paths are mirrored images ofone another.
 13. The monolithic coherent transceiver of claim 1, whereinthe CTM further comprises a plurality of 3 dB couplers that collectivelydistribute the second LO to each of the serpentine optical paths of eachof the first and second MZMs of each of the first and second IQmodulators.
 14. The monolithic coherent transceiver of claim 1, whereinthe electrical modulation signal comprises a set of PM-QAM or PM-QPSKsignals, and wherein at least a portion of the electrical modulationsignal are direct-current-coupled to the first MZM and the second MZM ofthe coherent transmitter module.
 15. A monolithic coherent transceiver,comprising: a first input port to receive an optical input, the opticalinput comprising a carrier modulated by a signal through a modulationscheme; a second input port to receive an optical local oscillation(LO), a frequency of the optical LO substantially equal to a frequencyof the carrier; a LO splitter to split the optical LO into a first LOand a second LO with a splitting ratio between the first LO and thesecond LO; a coherent receiver module (CRM) to detect the signal basedon the optical input and the first LO; a third input port to receive anelectrical modulation signal; a coherent transmitter module (CTM) togenerate an optical output signal based on the second LO and theelectrical modulation signal; and an output port to transmit the opticaloutput signal, wherein the modulation scheme comprises polarizationmultiplexed quadrature amplitude modulation (PM-QAM) or polarizationmultiplexed quadrature phase shift keying (PM-QPSK), wherein the LOsplitter comprises: a 1×2 optical coupler having an input, a firstoutput and a second output; a tunable phase shifter having an input andan output, the tunable phase shifter configured to provide a phase shiftbased on an electrical voltage; and a 2×2 optical coupler having a firstinput, a second input, a first output and a second output, and wherein:the 1×2 optical coupler is configured to receive the optical LO at theinput of the 1×2 optical coupler, the first output of the 1×2 opticalcoupler is coupled to the input of the tunable phase shifter, the outputof the tunable phase shifter is coupled to the first input of the 2×2optical coupler, the second output of the 1×2 optical coupler is coupledto the second input of the 2×2 optical coupler, the 2×2 optical coupleris configured to generate the first LO at the first output of the 2×2optical coupler, the 2×2 optical coupler is further configured togenerate the second LO at the second output of the 2×2 optical coupler,and the splitting ratio between the first LO and the second LO isdetermined by the phase shift based on the electrical voltage.
 16. Amonolithic coherent transceiver, comprising: a first input port toreceive an optical input, the optical input comprising a carriermodulated by a signal through a modulation scheme; a second input portto receive an optical local oscillation (LO), a frequency of the opticalLO substantially equal to a frequency of the carrier; a LO splitter tosplit the optical LO into a first LO and a second LO with a splittingratio between the first LO and the second LO; a coherent receiver module(CRM) to detect the signal based on the optical input and the first LO;a third input port to receive an electrical modulation signal; acoherent transmitter module (CTM) to generate an optical output signalbased on the second LO and the electrical modulation signal; and anoutput port to transmit the optical output signal, wherein themodulation scheme comprises polarization multiplexed quadratureamplitude modulation (PM-QAM) or polarization multiplexed quadraturephase shift keying (PM-QPSK), wherein the LO splitter comprises: apolarization beam rotator-splitter (PBRS) having an input, a firstoutput and a second output; a first 2×2 optical coupler having a firstinput, a second input, a first output and a second output; a tunablephase shifter having an input and an output, the tunable phase shifterconfigured to provide a phase shift based on an electrical voltage; anda second 2×2 optical coupler having a first input, a second input, afirst output and a second output, and wherein: the PBRS is configured toreceive the optical LO at the input of the PBRS, the first output of thePBRS is coupled to the first input of the first 2×2 optical coupler, thesecond output of the PBRS is coupled to the second input of the first2×2 optical coupler, the first output of the first 2×2 optical coupleris coupled to the input of the tunable phase shifter, the output of thetunable phase shifter is coupled to the first input of the second 2×2optical coupler, the second output of the first 2×2 optical coupler iscoupled to the second input of the second 2×2 optical coupler, thesecond 2×2 optical coupler is configured to generate the first LO at thefirst output of the second 2×2 optical coupler, the second 2×2 opticalcoupler is further configured to generate the second LO at the secondoutput of the second 2×2 optical coupler, and the splitting ratiobetween the first LO and the second LO is determined by the phase shiftbased on the electrical voltage.